It is known in the art to which this invention pertains to provide a variable density window comprising a pair of overlying light-polarizing elements wherein one of the light-polarizing elements is fixedly positioned and the other element is rotatable with respect thereto. Generally representing the prior art is U.S. Pat. No. 2,311,840 issued Feb. 23, 1943 in the name of Edwin H. Land. In the Land patent, hand-operated crank means are provided to drive the rotatable light-polarizing element. However, in such references as U.S. Pat. No. 3,521,300 dated July 21, 1970, a drive roller controlled by electrically energized motor means is also suggested in order to effect rotation of the movable light-polarizing element.
It has been observed that when using only a pair of polarizing elements, one stationary and the other rotatable, although almost complete extinction of the light transmitted by the variable density window is effected when the principal axes of polarization of the elements are at right angles to each other, it is still possible for an observer situated on one side of the window to see an object disposed on the other side, especially if viewing of the object is effected at an angle to the parallel planes of the polarizing elements. Such inconveniences and shortcomings of the prior art are completely eliminated by the structure of this invention utilizing a pair of spaced-apart stationary polarizing elements having their principal axes of polarization aligned, and placing the rotatable polarizing element between the stationary elements and spaced therefrom.